PROJECT SUMMARY/ABSTRACT Obesity and type-2 diabetes are both considered major risk factors for the development of multiple cardiovascular diseases worldwide. In the US alone, about 36% of the adult population are obese and over 25 million suffer from diabetes. Importantly, the prevalence of type-2 diabetes has been shown to dramatically increase with obesity. Surprisingly, while signs of lymphatic dysfunction have been reported in obesity and diabetes, the mechanisms underlying lymphatic dysfunction are unknown. Efficient lymph transport relies critically on the intrinsic spontaneous contractions of lymphatic muscle cells (LMCs). These contractions are initiated by action potentials that originate at a pacemaking site and then rapidly propagate between the strongly- coupled LMCs. In contrast to arterioles, electrical coupling between lymphatic endothelial cells (LECs) and LMCs through MEJs (myoendothelial junctions) appears to be quite limited; while this may be essential for the focal generation of pacemaking signals, preventing leak of depolarizing current into the endothelium, the idea has not been investigated. Thus, it is completely unknown whether there are any conducted signals in the lymphatic endothelium that can regulate LMC contractility and to what extent heterocellular cross-talk between LECs and LMCs plays a role in lymphatic function. The preliminary data obtained for this proposal show that: 1) signals can be conducted as Ca2+ waves in the lymphatic endothelium, and these waves can regulate the contractile function of LMCs; 2) in obese mice, the spontaneous contractions of isolated lymphatic vessels are impaired (i.e. decreased ejection fraction); 3) the serine protease inhibitor PAI-1, which regulates the formation of vascular MEJs, is an important biomarker for metabolic syndrome, diabetes, and obesity and is upregulated in lymphatic vessels from obese mice compared to those from control healthy mice; and 4) genetic overexpression of PAI-1 (tgPAI-1) in non-obese mice results in lymphatic vessels with impaired contractility, while vessels from age- matched PAI-1 KO mice (PAI-1-/-) show improved lymphatic function. These results suggest novel endothelium- dependent mechanisms through which lymphatic contractions can be regulated, associate obesity with lymphatic contractile dysfunction, point to a novel, critical role for PAI-1 in regulating lymphatic function, and suggest its potential use as therapeutic target to ameliorate lymphatic dysfunction associated with obesity. Therefore, the hypotheses to be tested are: a) Conducted signals in the lymphatic endothelium can regulate LMC contractility through either direct LEC-LMC coupling via MEJs and/or diffusion of released factors; b) increased levels of PAI- 1 result in abnormal cellular interactions, upregulating the formation of MEJs; c) PAI-1 upregulation associated with obesity results in impaired lymphatic function through endothelium-dependent mechanisms; and, d) PAI-1 inhibition can ameliorate lymphatic function in obesity. The studies here proposed and their outcome will expand our understanding on the pathophysiological mechanisms leading to lymphatic dysfunction in metabolic syndrome and diabetes associated with obesity.